diff --git a/spring-orm/src/main/java/org/springframework/orm/jpa/vendor/EclipseLinkJpaDialect.java b/spring-orm/src/main/java/org/springframework/orm/jpa/vendor/EclipseLinkJpaDialect.java
index c26b9bb27b..5b313816d1 100644
--- a/spring-orm/src/main/java/org/springframework/orm/jpa/vendor/EclipseLinkJpaDialect.java
+++ b/spring-orm/src/main/java/org/springframework/orm/jpa/vendor/EclipseLinkJpaDialect.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2023 the original author or authors.
+ * Copyright 2002-2024 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -18,6 +18,8 @@ package org.springframework.orm.jpa.vendor;
 
 import java.sql.Connection;
 import java.sql.SQLException;
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;
 
 import jakarta.persistence.EntityManager;
 import jakarta.persistence.PersistenceException;
@@ -39,14 +41,18 @@ import org.springframework.transaction.TransactionException;
  * JDBC and JPA operations in the same transaction, with cross visibility of
  * their impact. If this is not needed, set the "lazyDatabaseTransaction" flag to
  * {@code true} or consistently declare all affected transactions as read-only.
- * As of Spring 4.1.2, this will reliably avoid early JDBC Connection retrieval
- * and therefore keep EclipseLink in shared cache mode.
+ * This will reliably avoid early JDBC Connection retrieval and therefore keep
+ * EclipseLink in shared cache mode.
  *
  * <p><b>NOTE: This dialect supports custom isolation levels with limitations.</b>
- * Consistent isolation level handling is only guaranteed when all Spring transaction
- * definitions specify a concrete isolation level, and as of 6.0.10 also when using
- * the default isolation level with non-readOnly and non-lazy transactions. See the
+ * Consistent isolation level handling is only guaranteed when all Spring
+ * transaction definitions specify a concrete isolation level and when using the
+ * default isolation level with non-readOnly and non-lazy transactions; see the
  * {@link #setLazyDatabaseTransaction "lazyDatabaseTransaction" javadoc} for details.
+ * Internal locking happens for transaction isolation management in EclipseLink's
+ * DatabaseLogin, at the granularity of the {@code EclipseLinkJpaDialect} instance;
+ * for independent persistence units with different target databases, use distinct
+ * {@code EclipseLinkJpaDialect} instances in order to minimize the locking impact.
  *
  * @author Juergen Hoeller
  * @since 2.5.2
@@ -58,6 +64,8 @@ public class EclipseLinkJpaDialect extends DefaultJpaDialect {
 
 	private boolean lazyDatabaseTransaction = false;
 
+	private final Lock transactionIsolationLock = new ReentrantLock();
+
 
 	/**
 	 * Set whether to lazily start a database resource transaction within a
@@ -94,13 +102,13 @@ public class EclipseLinkJpaDialect extends DefaultJpaDialect {
 
 		int currentIsolationLevel = definition.getIsolationLevel();
 		if (currentIsolationLevel != TransactionDefinition.ISOLATION_DEFAULT) {
-			// Pass custom isolation level on to EclipseLink's DatabaseLogin configuration
-			// (since Spring 4.1.2 / revised in 5.3.28)
+			// Pass custom isolation level on to EclipseLink's DatabaseLogin configuration.
 			UnitOfWork uow = entityManager.unwrap(UnitOfWork.class);
 			DatabaseLogin databaseLogin = uow.getLogin();
-			// Synchronize on shared DatabaseLogin instance for consistent isolation level
+			// Lock around shared DatabaseLogin instance for consistent isolation level
 			// set and reset in case of concurrent transactions with different isolation.
-			synchronized (databaseLogin) {
+			this.transactionIsolationLock.lock();
+			try {
 				int originalIsolationLevel = databaseLogin.getTransactionIsolation();
 				// Apply current isolation level value, if necessary.
 				if (currentIsolationLevel != originalIsolationLevel) {
@@ -116,20 +124,26 @@ public class EclipseLinkJpaDialect extends DefaultJpaDialect {
 					databaseLogin.setTransactionIsolation(originalIsolationLevel);
 				}
 			}
+			finally {
+				this.transactionIsolationLock.unlock();
+			}
 		}
 		else if (!definition.isReadOnly() && !this.lazyDatabaseTransaction) {
 			// Begin an early transaction to force EclipseLink to get a JDBC Connection
 			// so that Spring can manage transactions with JDBC as well as EclipseLink.
 			UnitOfWork uow = entityManager.unwrap(UnitOfWork.class);
-			DatabaseLogin databaseLogin = uow.getLogin();
-			// Synchronize on shared DatabaseLogin instance for consistently picking up
+			// Lock around shared DatabaseLogin instance for consistently picking up
 			// the default isolation level even in case of concurrent transactions with
-			// a custom isolation level (see above), as of 6.0.10
-			synchronized (databaseLogin) {
+			// a custom isolation level (see above).
+			this.transactionIsolationLock.lock();
+			try {
 				entityManager.getTransaction().begin();
 				uow.beginEarlyTransaction();
 				entityManager.unwrap(Connection.class);
 			}
+			finally {
+				this.transactionIsolationLock.unlock();
+			}
 		}
 		else {
 			// Regular transaction begin with lazy database transaction.
@@ -143,9 +157,6 @@ public class EclipseLinkJpaDialect extends DefaultJpaDialect {
 	public ConnectionHandle getJdbcConnection(EntityManager entityManager, boolean readOnly)
 			throws PersistenceException, SQLException {
 
-		// As of Spring 4.1.2, we're using a custom ConnectionHandle for lazy retrieval
-		// of the underlying Connection (allowing for deferred internal transaction begin
-		// within the EclipseLink EntityManager)
 		return new EclipseLinkConnectionHandle(entityManager);
 	}